The present invention relates generally to providing an apparatus and method for electropolishing products made from metals, and in particular, electropolishing metallic medical devices such as stents, made of stainless steel, titanium, tungsten, nickel-titanium, tantalum, cobalt-chromium-tungsten, etc. While the apparatus and method are described herein as being applicable mainly to medical stents, in particular intravascular stents, the invention is not limited to such medical products. For example, the methods may be applied to electropolish metallic automotive or aerospace components.
Stents are generally tube-shaped intravascular devices placed within a blood vessel to maintain the patency of the vessel and, in some cases, to reduce the development of restenosis. The stents may be formed in a variety of configurations which are typically expandable since they are delivered in a compressed form to the desired site. Such a configuration may be a helically wound wire, wire mesh, weaved wire, serpentine stent, or a chain of rings. The walls of stents are typically perforated in a framework design of wire-like connected elements or struts or in a weave design of cross-threaded wire. Some stents are made of more than one material. The stent may be, for example, a sandwich of metals having outer layers of a biocompatible material, such as stainless steel, with an inner layer providing the radioopacity to the stent needed for tracking by imaging devices during placement. A stent made of such material may be, for example, a thin layer of titanium between layers of stainless steel. In forming such stents from metal, a roughened outer surface of the stent may result from the manufacturing process. It is desirable for the surface of the stent to be smooth so that it can be easily inserted and traversed with low friction through the blood vessels toward the site of implantation. A rough outer surface may not only cause increased frictional obstruction, but may also damage the lining of the vessel wall during insertion. Furthermore, smooth surfaces decrease the probability of thrombogenesis and corrosion.
Since the processing to form metallic stents often results in a product initially having undesirable burrs, sharp ends or debris and slag material from melting the metal during processing, as a first order treatment of the product, descaling of the surface is required in preparation of further surface treatment such as electropolishing.
The present invention is directed to an apparatus and method for electropolishing such stents after they have been descaled by an appropriate method, such as that disclosed in concurrently filed application Ser. No. 11/370,660. Descaling may include, for example, dipping the stent into a strongly acidic solution and thereafter ultrasonically cleaning the stent.
Electropolishing is an electrochemical process by which some of the surface metal is electrolytically dissolved. In general, the metal stent serves as an anode and is connected to a power supply while immersed in an electrolytic solution having a metal cathode connected to the negative terminal of the power supply. Current therefore flows from the stent, as the anode, causing it to become polarized. The rate at which the metal ions on the stent are dissolved is controlled by the applied and/or voltage. The positioning of the cathode relative to the stents is important so that there is an even distribution of current to the stent. According to the theory of electropolishing, the current density is highest at high points protruding from a surface and is lowest at the surface low points. Thus, the higher current density at the raised points causes the metal to dissolve faster at these points which thus levels the surface. Electropolishing therefore serves to smooth the surface, typically to the point where it is shiny and reflective.
The present invention provides an apparatus and process for electropolishing a plurality of metallic devices such as stents simultaneously to consistently produce smooth surfaces.